Two Arlington men were injured yesterday, one critically, when the aluminum ladder they were using while trimming tree branches in the 2300 block of N. Quincy Street hit a power line.

Richard Diethorn, 18, of 3590 N. 14th St., was taken by helicopter to the Washington Hospital Center's Medstar trauma unit, where a spokeswoman said he was in the burn intensive-care unit in critical condition.

The second man, identified by neighbors as Chris Lane, 21, of 1053 N. Monroe St., was listed in fair condition last night at Arlington Hospital. A hospital spokeswoman said he was being kept overnight for observation.

"I was in my house . . . and I saw a horrible flash . . . . It was a bang and then a flash of light," said Marian Bensen, who was having her trees trimmed when the accident occurred. She said the two men did odd jobs in the neighborhood.

Another neighbor, Flo Rosse, said D.C. Fire Capt. Paul Jackson, who was painting her house, across the street from the accident, administered artificial respiration to the men, and revived Diethorn.

W.D. Keesee, a Virginia Power Co. official who was supervising the repair of the electrical lines on the street after the accident, said the company has a policy of cutting tree branches that are interfering with power lines for no charge.